Radical Ions of 3-Styryl-quinoxalin-2-one derivatives studied by pulse radiolysis in organic solvents

Abstract

The absorption-spectral and kinetic behaviors of radical ions and neutral hydrogenated radicals of seven 3-styryl-quinoxalin-2(1 H)-one (3-SQ) derivatives, one without substituents in the styryl moiety, four others with electron-donating (R = -CH3, -OCH3, and -N(CH3)(2)) or electron-withdrawing (R = -OCF3) substituents in the para position in their benzene ring, and remaining two with double methoxy substituents (-OCH3), however, at different positions (meta/para and ortho/meta) have been studied by UV-vis spectrophotometric pulse radiolysis in neat acetonitrile saturated with argon (Ar) and oxygen (O-2) and in 2-propanol saturated with Ar, at room temperature. In acetonitrile solutions, the radical anions (4R-SQ(center dot-)) are characterized by two absorption maxima located at lambda(max) = 470-490 nm and lambda(max) = 510-540 nm, with the respective molar absorption coefficients epsilon(470-490) = 8500-13 100 M-1 cm(-1) and epsilon(510-540) = 6100-10 300 M-1 cm(-1), depending on the substituent (R). All 4R-SQ(center dot-) decay in acetonitrile via first-order kinetics, with the rate constants in the range (1.2-1.5) x 10(6) s(-1). In 2-propanol solutions, they decay predominantly through protonation by the solvent, forming neutral hydrogenated radicals (4R-SQH(center dot)), which are characterized by weak absorption bands with lambda(max) = 480-490 nm. Being oxygen-insensitive, the radical cations (4R-SQ(center dot+)) are characterized by a strong absorption with lambda(max) = 450-630 nm, depending on the substituent (R). They are formed in a charge-transfer reaction between a radical cation derived from acetonitrile (ACN(center dot+)) and substituted 3-styryl-quinoxalin-2-one derivatives (4R-SQ) with a pseudo-first-order rate constant k = (2.7-4.7) x 10(5) s(-1) measured in solutions containing 0.1 mM 4R-3-SQ. The Hammett equation plot gave a very small negative slope (rho = -0.08), indicating a very weak influence of the substituents in the benzene ring on the rate of charge-transfer reaction. The decay of 4R-SQ(center dot+) in Ar-saturated acetonitrile solutions occurs with a pseudo-first-order rate constant k = (1.6-6.2) x 10(4) s(-1) and, in principle, is not affected by the presence of O-2, suggesting charge-spin delocalization over the whole 3-SQ molecule. Most of the radiolytically generated transient spectra are reasonably well-reproduced by semiempirical PM3-ZINDO/S (for 4R-SQ(center dot-)) and density functional theory quantum mechanics calculations employing M06-2x hybrid functional together with the def2-TZVP basis set (for 4R-SQ(center dot+)).